Transportation system, such as vehicles, may be powered partially or completely by electrical motors with energy stored in rechargeable batteries. Likewise, non-transportation devices may similarly be electric powered with rechargeable batteries a source of power. Many hybrid vehicles in use today in the United States and around the world use Nickel Metal Hydride high voltage hybrid traction batteries (NiMH). Other battery types commonly used are lead acid and lithium amongst others. While we will focus our discussion on NiMH cells, all batteries suffer from similar problems to different degrees. Multiple individual NiMH cells are combined to form the high-voltage battery ‘pack’ that is used to assist in vehicle propulsion and improve fuel economy. NiMH batteries normally operate in a range of 1.2-1.4 V per cell. Multiple cells are combined in series to produce the high voltage required by the vehicle. For example, the 1999-2003 Toyota Prius uses 228 individual cells (combined into six-cell modules−38 modules per pack) for a peak operating voltage of approx. 330 volts; the 2004-2009 Prius uses 168 individual cells (combined into six-cell modules−28 modules per pack) to produce a peak operating voltage of approximately 243 volts. NiMH cells are designed to have low internal resistance and low self-discharge rates, which combined with low manufacturing costs, have made them the technology of choice for hybrid vehicles during the last decade.
During normal vehicle operations, the hybrid battery cells are charged and discharged via normal vehicle use. As a result of this use, the battery cells may deteriorate over time as their lifespan is consumed. Every NiMH cell is unique and deteriorates at a slightly different rate than the other cells within the same hybrid vehicle battery pack. This is based on a variety of factors including but not limited to: Each cell's internal resistance when new & degradation over time. Each cell's self-discharge rate when new & over time may vary from the other cells in the same hybrid battery pack. The physical location of the cell within the battery pack may influence the rate of degradation—cells in the center of the hybrid battery pack often degrade more rapidly than cells on the perimeter of the battery pack. The baseline ambient temperatures of the battery pack (i.e. climate in which the vehicle operates) influences the speed at which the various factors cited may influence the cells. There are additional factors that influence the variation in deterioration rate between NiMH cells within a hybrid battery pack, the above list is not meant to be conclusive. These factors combine to allow the hybrid battery cell's voltage to drift out of sync over time. As a result the hybrid battery pack becomes ‘imbalanced’ with different cells being charged to different voltages.
When the imbalance between NiMH hybrid battery cells becomes large enough, most vehicles include circuits which may display a fault code. Whether the battery is, in fact, unusable or not, may be irrelevant if the diagnostics circuits within the vehicle think it is. The vehicle may display an “incurable” battery fault. The vehicle owner's current remedy options are to replace the entire battery pack as a unit or disassemble the hybrid battery pack into the individual six cell modules and recharge/replace failed cells or modules at the cell/module level.
Vehicle manufacturers also seem to determine the health of an entire battery pack by looking at the health of the strongest cell. Even though a cell may have a high voltage, it may produce little current because of accumulated resistance therein. The onboard computer in the vehicle will still perceive the battery as OK when it provides little benefit. Both circumstances are undesirable, though this one is perhaps worse because the user does not know why the battery appears Ok but fuel consumption is increased.
Practically speaking, the vehicle manufacture would prefer full replacement of the entire battery pack, which is faster than identifying weak cells. No system is provided to repair cells in place.
It is known to discharge and recharge batteries for reconditioning, but the known circuits are elemental and typical charge and discharge at fixed rates. Recharge is slow to prevent battery overheating. Use of a cooling fan at a fixed speed is also known.
Discharging has been done using incandescent lamps to run down the batteries. Such lamps have no intelligence to optimize discharge rates.